Although not exclusively, the invention applies more particularly to correcting the hearing problems of a patient and the device of the invention can thus be used as a portable hearing aid.
Generally speaking, conventional hearing aids comprise an input microphone, amplification means and an output earpiece. Amplification is embodied throughout the entire spectrum of frequencies. Firstly, this proves to be superfluous as, for each individual patient, specific zones of the spectrum are best conserved, and secondly may be harmful owing to the fact that accordingly no differentiation exists between the "useful" frequencies and the other frequencies.
Furthermore, it appears that, as regards certain pathological ears, a loss of hearing is expressed by an inability of the ear, not of actually being able to hear, but of being unable to correctly identify the sound emitted. Moreover, clinical tests show that this inability to correctly identify sounds is more or less marked according to the zones of frequencies involved and varies according to the type and form of the loss of hearing particular to each patient. In a seriously affected hearing zone, a sound signal having given frequencies may then be received as a signal presenting different frequencies and, as a result, the sound "heard" by the patient may appear to differ from the sound transmitted. For example, the patient will hear an "S" for an "F", and vice versa. Owing to this, a sound signal, even if amplified, still risks being poorly received by the pathological ear which somehow distorts the sound transmitted, regardless of the intensity of said sound.
Thus, the frequencies of the sound spectrum have been considered as being shifted by a whole octave number equal to at least one. (It is clearly understood that the term "octave" as used here represents the interval of two vibrations, one of which has a frequency twice the other vibration). The frequencies of the signal may then occur in a more favorable zone, namely the best preserved hearing zone of the pathological ear. Furthermore, as the frequencies of the signal are shifted by one octave, the corresponding sound shall still be able to be recognised, even if it is noticed as being more low-pitched or more high-pitched.
However, in this case, all the frequencies of the spectrum also undergo this shift which, in the last analysis, results in drawbacks similar to those indicated previously.